This invention is concerned with bandwidth compression for television video signals.
If High Definition Television (HDTV) is to become a practical proposition, then some method of transmitting it to the home is required. It is widely assumed that such transmissions will be via satellite, since spare radio spectrum is not available elsewhere. The principal problem in this case is the extremely wide bandwith of the original HDTV signal--possibly in the region of 40 to 50 MHz for the combined luminance and chrominance information. Such bandwidths can not be accommodated in the 12GHz DBS (digital broadcast by Satellite) band.
The high transmission bandwidth required for HDTV will cause problems not only for terrestrial and satellite broadcasting but also for signal dissemination by other media such as videotape, videodisc and cable. Some form of bandwidth reduction is required in order to overcome these difficulties.
Methods of bandwidth reduction have been described which use sub-Nyquist sampling with ore-filtering in one, two and three dimensions. More recently motion adaptive pre-filtering techniques have been described, are for example NHK Laboratory Note No. 304, 1984, NINOMIYA et al "A single Channel HDTV Broadcast System" and a paper presented by FUJIO, SUGIMOTO and NINOMIYA at the 14th International Television Symposium, Montreux, June 1985, "HDTV Transmission Method (MUSE)". This system is based upon the removal, by filtering, of image frequency components that are assumed to be of little use to the eye. The filtered signal has a much reduced bandwidth and can be re-sampled at a lower rate for transmission.
The use of motion-adaptive spatial sub-sampling can yield impressive reductions in transmitted bandwidth. Still areas of the picture are transmitted at full resolution but with the information being distributed over many different television fields. Moving areas are transmitted at a reduced resolution, taking advantage of the eye's alleged inability to perceive detail in moving objects. This is the approach used in the NHK's MUSE system (described in the above papers) and which has an objective bandwidth compression factor of 4:1, leading to a final transmission bandwidth of only 8.1 MHz. This system has given results that, while in general encouraging, nevertheless can be variable and unpredictable. We have appreciated that the system performance will differ from receiver to receiver and also vary as reception conditions, and hence the carrier to noise ratio, changes, and that this arises because the system detects which areas of the scene are moving and which stationary at every receiver. Furthermore, the system is relatively expensive in requiring movement detection circuitry at every receiver.